If none of that works you will have to disassemble your device and check for broken, disconnected wires. check for bad capacitors (bulging, leaking). you want to go ahead and past some images of your circuit boards. Use this guide Adding images to an existing question which will allow us to see what you see.

I have the same keyboard (bought used) and have several keys that have to be pressed really hard to sound off. I have a feeling it is dust or corrosion or something that is retarding the contact between the keys and whatever it is supposed to activate. Should I blow compressed air or electrical contact cleaner or something onto the keys or do I need to take the keyboard apart to try to determine the problem?

Resources

Repairability

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It's time to speak out for your right to repair

We have a chance to guarantee our right to repair electronic
equipment—like smartphones, computers, and even farm equipment. This is a once-in-a-generation
chance to protect local repair jobs—the corner mom-and-pop repair shops that
keep getting squeezed out by manufacturers.

Join the cause and tell your state representative to support Right to Repair. Tell them you believe repair should be fair, affordable, and accessible. Stand up for your right to repair!